The cease-fire covering three war-torn provinces in southern Syria is the first tangible outcome following months of strategy and diplomacy between the new Trump administration and Russian President Vladimir Putin's Moscow.

Trump tweeted that the cease-fire, which came into effect at noon Sunday, "will save lives."

"Now it is time to move forward in working constructively with Russia!" he posted on Twitter shortly after the agreement came into effect.

A resident and local opposition activist in Daraa, near the Jordanian border, reported an uneasy calm hours into the truce.

"There's still a lot of anxiety," said Ahmad al-Masalmeh. "We've entered the cease-fire but there are no mechanisms to enforce it. That's what concerns people."

Six years of fighting and siege have devastated Daraa, one of the first cities to see large protests against President Bashar Assad in 2011.

It remains contested by U.S.-backed rebels and Syrian government forces supported by Russia and Iran. Large swaths of the city have been reduced to rubble by government artillery and Russian air power.

The truce also covers the Quneitra and Sweida provinces, where the government and the rebels are also fighting Islamic State militants, who are not included in the agreement.

The Britain-based Syrian Observatory for Human Rights, which monitors the conflict through a network of on-the-ground activists, reported calm across the three provinces as dusk fell Sunday.

The cease-fire agreement followed weeks of secretive talks between the U.S., Russia and Jordan in Amman to address the buildup of Iranian-backed forces, in support of the Syrian government, near the Jordanian and Israeli borders.

Israel has repeatedly said it would not allow Iran, which is a close ally of the Syrian government, to set up a permanent presence in Syria. It has carried out a number of airstrikes in Syria against suspected shipments of "game-changing" weapons bound for Hezbollah in Lebanon.

It has also struck Syrian military installations on several occasions this year after shells landed inside the Israeli-controlled side of the Golan Heights.

Prime Minister Benjamin Netanyahu said Sunday that Israel would welcome a "genuine cease-fire" in southern Syria so long as it doesn't enable Iran and its proxies to develop a military presence along the border.

The Trump administration also ordered airstrikes against the Syrian government and Iranian-backed militias, in a break with Obama administration policy. The strikes, including one on a government air base in central Syria, drew only muted responses from Moscow.

No cease-fire has lasted long in the six-year-old Syrian war, and no mechanisms have been publicly set out to monitor or enforce this latest endeavor.

It was announced Thursday on the sidelines of the G-20 summit in Hamburg after a meeting between Trump, Putin and their top diplomats.

The Syrian government maintains it is fighting a war against terrorist groups. The Al-Qaida-linked Levant Liberation Committee is one of the most effective factions fighting alongside rebels in Daraa.